In the gaseous phase synthesis of diamond, a number of methods have been developed, for example, a hot filament CVD method comprising utilizing a thermoelectron radiation material heated at a high temperature to decompose and activate a raw material gas, a microwave plasma CVD method comprising utilizing microwave plasma, a DC plasma CVD method comprising utilizing DC plasma, a dia-jet method comprising utilizing a DC plasma torch, a burner method comprising utilizing an oxygen-acetylene combustion flame, etc. Above all, the hot filament CVD method and the microwave plasma CVD method are typical methods.
According to the hot filament CVD method comprising utilizing a thermoelectron radiation material heated at a high temperature to decompose and activate a raw material gas, it is possible to enlarge a coating zone by designing the shape of a filament more readily than in the other plasma utilizing process. Even if the thermoelectron radiation material, as an exciting source, is enlarged, however, the ordinary apparatus construction is so composed that a substrate is arranged on one plane to face the thermoelectronic emission material and the utilization efficiency of the space relative to the thermoelectron radiation material is low so that the diamond-forming area is markedly limited relatively to the capacity of the exciting source. Since the size of the thermoelectron radiation material is generally smaller relatively to the size of a reaction vessel and diamond is formed only in the vicinity of the thermoelectron radiation material, the dead zone in a reaction vessel is large and the diamond-forming area is small relatively to the occupied area of the apparatus, so that the apparatus is not suitable for use as a production apparatus on a commercial scale. The flow of a raw material gas is governed by convection and is not simple near the thermoelectron radiation material because of using a reaction pressure of several ten to several hundred Torr and accordingly, effective feeding of the raw material gas and exhausting of the reacted gas cannot well be carried out.
In the hot filament CVD method, therefore, it has eagerly been desired to develop a process for the synthesis of diamond and an apparatus for practicing this process, wherein a large sized thermoelectron radiation material is provided as an exciting source and a substrate is arranged with a high space efficiency relatively to the thermoelectron radiation material, so that the capacity of the exciting source can be displayed to the maximum, the utility efficiency of a raw material gas in a reaction zone can be increased, the dead zone in a reaction vessel can be decreased and the occupied area of the synthesis apparatus can be reduced per the diamond-forming area, thus obtaining excellent productivity with a compact size of apparatus. The present invention has been made so as to respond to the desirement.
In various synthesis techniques of diamond, the inventors have made various studies to improve the hot filament CVD method and apparatus capable of enlarging the diamond-forming area in relatively easy manner and to utilize effectively the capacity of a thermoelectron radiation material, and consequently, have reached a process and apparatus for producing diamond with excellent productivity as well as a compact size of apparatus, which can be applied to production on a commercial scale. The present invention can thus be accomplished.